Reference for Pesachim 121:9

יכול שחטו שלא למנויו יהא כעובר על המצוה וכשר ת"ל במכסת תכוסו הכתוב שנה עליו לעכב

How do we know this? Because our Rabbis taught, [Then shall he and his neighbour next unto him take one] according to the number of [be-miksath] [the souls]:<span class="x" onmousemove="('comment',' Ex. XII, 4.');">¹⁰</span> this teaches that the Paschal lamb is not slaughtered save for those who are registered [numbered] for it.

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